Grub damage vs fungus: how to tell the difference

You walk out to brown patches in your lawn and your first instinct is to grab a fungicide. But if the problem is actually grubs, that fungicide does nothing and you lose more lawn while misdiagnosing the issue. Here is how to tell the difference quickly.

The tug test

Grab a handful of the brown grass and pull. If the turf peels up like a carpet with no resistance, you have grubs. They have eaten the roots, so the grass has nothing anchoring it. If the grass resists pulling and the roots are intact, it is likely fungal disease.

Check the edge pattern

Fungal diseases typically create circular or semi-circular patterns with a distinct border. Brown patch has a smoke ring edge. Dollar spot creates small defined circles. Grub damage is irregular — the edges are ragged and the damaged area does not have a clear shape or border.

Look underground

Pull back a section of sod in the damaged area. If you see white, C-shaped grubs (Japanese beetle larvae) in the soil, that is your answer. More than 5-6 per square foot means treatment is needed. If you see no grubs and the roots look brown or mushy, it is likely disease.

Timing clues

Grub damage typically appears in late August through October when the larvae are actively feeding. Fungal diseases have different timing: brown patch hits in summer heat, dollar spot in spring and fall, and snow mold in early spring. If your brown patches appeared in September and the grass peels up, start with the grub assumption.

Treatment

For grubs: apply a curative product with trichlorfon or carbaryl for active infestations. Next year, apply a preventive product (chlorantraniliprole) in late spring before the beetles lay eggs. For fungus: identify which disease, adjust watering and mowing, and apply fungicide if spreading.

Not sure what you are looking at? Scan it with Grasslytics. The AI can distinguish between grub damage, fungal disease, drought stress, and other common causes.